Toward Fundamental Understanding of Stress Corrosion Cracking in Gas Metal Arc Welding of High-Strength Aluminum Alloy 7003 with 5356 Filler Metal
The stress corrosion cracking (SCC) phenomenon was investigated in high-strength aluminum alloy (AA) welds. A weld region, significant to SCC, was discovered to exist near the weld toes and is termed the fused-overlap zone (FOZ). SCC failure initiation was determined to be strongly related to the existence of large precipitates (1-20 µm in diameter) within the FOZ. SCC failure initiations in welds made of AA 7003 base metal and AA 5356 filler wire consistently correspond with the FOZ that contains grain boundary precipitation identified as T phase, (Al, Zn)49Mg32, by TEM and EDS studies. Since T phase is electrochemically anodic to the Al-matrix, precipitates in the FOZ are preferentially dissolved when the weldment is exposed to salt water; this leads to a more aggressive solution within the pitting/intergranular (IG) corrosion generated by the dissolving network of T precipitation. When the weldment is additionally exposed to a tensile stress across the FOZ, mechanical stress intensities arise at the pitting/IG corrosion. It is theorized that if the pits/crevices extend into the SCC-susceptible heat affected zone (HAZ) of the base metal, and the crack length is long enough for the effective stress intensity (KI) to be above the threshold stress intensity (KISCC), SCC will initiate.
Variables other than the network of T phase precipitation, such as, grain size, surface geometry, and HAZ grain boundary precipitation were also investigated. HAZ precipitation and grain size were determined to contribute towards SCC propagation. Surface geometry contributes to SCC initiation.
The network of T phase precipitation occurs because, prior to solidification, the FOZ is enriched with Mg. The Mg enrichment of the FOZ is best understood by first knowing the fact that, during welding using AA 5356, Mg2+ vaporizes from the filler wire and weld pool. These Mg2+ ions combine with other vaporized elements and form phases that coat all nearby surfaces, including the base metal surface immediately adjacent to the weld fusion zone.
During welding, the molten weld pool forms the FOZ by wetting outward over the base metal surface that is coated with Mg-rich surface phases. It is theorized that the surface phases are dissolved into the wetting FOZ and enrich it with Mg. The FOZ solidifies rapidly enough to eliminate the opportunity for weld convection currents to mix the Mg enrichment into the bulk fusion zone. The solidification of the FOZ segregates the Mg content to the grain boundaries where it forms the network of T phase precipitation.
It is found that the FOZ enrichment phenomenon is not isolated to the combination of AA 7003 base metal and AA 5356 filler wire. The breadth of this phenomenon extends into many different base metal AA systems (1xxx, 2xxx, 5xxx, and 6xxx), filler wires (5025, 5087, and 4043), joint geometries, heat inputs, power supplies, and welding processes (GTAW).
The breadth of the enriched FOZ phenomenon poses general implications beyond that of SCC; for example, a reduction in fatigue life is expected due to preferential pitting or IG corrosion in the FOZ. Engineering solutions to corrosion within the FOZ are also developed.
Industry Sponsor: Honda R&D Americas
Faculty Advisor: Wei Zhang (OSU)
Graduate Student: Tyler E. Borchers
Industry Contacts: Alan Seid, Pat Shafer